1. Field of the Invention
The present invention relates generally to the field of transmissions for multi-speed bicycles, and more particularly, relates to an automatic derailleur shifter featuring an improved hydraulic cylinder having a floating piston and means to precisely control the rate of fluid passage through and about the piston.
2. Description of the Prior Art
As set forth in U.S. Pat. Nos. 4,599,079 and 4,693,700, which patents are owned by the assignee of the presention application, a shifting device for derailleurs has been disclosed which features a small hydraulic cylinder for use as a stabilizing means to control and slow the pivotal movement of the movable shifter arm. Cylinders including hydraulic cylinders and pneumatic cylinders of various and numerous designs and configurations are known and have long been developed by prior workers in the art.
Experience has shown that while the design of the previously disclosed hydraulic cylinder is technically and practically correct, the efficient manufacture of this particular design has proved to be relatively difficult. Because of this, the precise timing and control of the piston movement has presented operating problems when the currently available automatic derailleur shifters were placed in use.
Specifically, the function of the hydraulic cylinder in the automatic derailleur shifter system is dependant upon the rather precisely controlled flow of hydraulic fluid through and about the piston as the piston is moved relative to the cylinder. In a small hydraulic cylinder of the type designed for use with an automatic derailleur shifter, it has been found that the correct flow of the hydraulic fluid can be properly controlled within the predetermined parameters only when the parts are accurately made within the designed tolerances. When the required tolerances cannot be maintained either through precise manufacturing techniques or by elimination of out-sized parts through extensive quality control procedures, the proper functioning of the hydraulic cylinder cannot be assured. In fact, it has been found that proper functioning hydraulic cylinders could be produced only when the parts either were made by hand and each part individually measured or when the parts were mass produced and then subjected to 100% quality control measurements and checking. Either of these procedures can greatly increase the cost of the finished part and has made the expense of producing a properly functioning hydraulic cylinder disproportionate to the expense of the automatic derailleur shifter in its entirety.
Accordingly, the need remains to provide a hydraulic cylinder suitable for use with an automatic derailleur shifter and for other associations that is capable of being mass produced without consequent loss of reliability and functionability and without requiring greatly increased or unacceptably expensive quality control costs.